Method and device for transmitting data on at least one electrical power supply line

ABSTRACT

The orthogonal frequency division multiplexing (OFDM) method is well-known for transmitting data on electrical power supply lines. According to this method, the items of information to be transmitted are distributed among numerous carriers, and the composite signal of the modulated carrier signals is transmitted in the form of an OFDM block. Standard OFDM methods are, however, highly sensitive to strong periodic pulse jammers. According to the invention, the method is thus devised such that the OFDM blocks to be transmitted have a length of approximately 85% of the interval between two periodic disturbing pulses. The carrier interval accordingly results from the reciprocal duration of the OFDM blocks. The transmitted OFDM blocks are synchronized with pulse-shaped periodic jammers in such a manner that one block at a time is located between two disturbing pulses. The pulse-shaped jammers can be gated at the receiver. To this end, the inventive device comprises an appropriately designed transmitter ( 20 ) and an associated receiver ( 30 ).

[0001] Method and device for transmitting data on at least oneelectrical power supply line

[0002] The invention relates to a method for transmitting data on atleast one electrical power supply line, on which pulse-shapedinterference signals occur periodically due to commutation processes,e.g. in rectifiers, with intermediate, almost interference-free timeintervals, using the OFDM method in which the information to betransmitted is distributed over a number of carriers and the compositesignal of all modulated carriers is transmitted in the form of a burst.In addition, the invention also relates to the associated device forcarrying out the method. The method and the device should be applicable,in particular but not exclusively, in direct-voltage railroad powersupplies.

[0003] It is known to transmit data on electrical power lines. Such atype of data transmission is of particular use especially in trafficengineering in order to implement, e.g. train control or signalling.

[0004] In the prior art broad-band transmission methods are used fortransmitting data on electrical power lines. The favorite method isgenerally the OFDM (orthogonal frequency division multiplex) methodwhich distributes the information to be transmitted over a very largenumber of mutually orthogonal carriers and transmits the compositesignal of all modulated carriers in the form of a so-called OFDM burst.Accordingly, filters used for suppressing noise must have a band passcharacteristic and the width of the band pass corresponds to at leastthe necessary band width of the composite OFDM signal. From EP 1 018 826A2, in particular, multicarrier transmission on power systems by usingan OFDM method is known in which the signal obtained by an inverseFourier transform and corresponding to a single data burst istransmitted for a predetermined time duration (OFDM burst duration) to areceiver, these bursts following one another as closed, i.e. temporarilycontiguous signal bursts. It is proposed to multiply each signal by awindow function or to use a specific digital filter for the purpose ofspectral limitation before the transmission.

[0005] Furthermore, in U.S. Pat. No. 4,845,466 A, a system for digitalhigh-speed information transmission in environments containinginterference signals is described and a specific solution is describedespecially for alternating-current transmission lines, in which firstthe zero transition of the alternating voltage is determined by means ofa sign detector and signal patterns for the noise pulses are generatedwith respect to the zero transition in order to enable the digital datato be transmitted to be gated out during the occurrence of the noisepulses. This solution is only specific for alternating-voltage powersystems and cannot be used with direct-current supplies, on the onehand, and, on the other hand, it cannot be used at all with OFDM sincethe OFDM bursts mentioned must always have a fixed duration and cannotbe interrupted—e.g. at the positions of noise pulses. It is especiallyin direct-current supplies that, generally due to the generation of thedirect voltage by means of rectifiers, in particular, periodicinterference signals, i.e. interference signals with preciselypredetermined position in time, occur which will be called “interferers”in brief or generally noise pulses in the text which follows.

[0006] The periodic noise pulses are caused by the commutation of thecurrent in the rectifier. The power converter components or valves,usually diodes or thyristors, in a rectifier conduct the direct currentalternately. When the direct current changes from one valve to anothervalve, inductances between the valves create a voltage peak which isassociated with voltage rises of the order of magnitude of about 400kV/s. Since the commutation times are determined by the frequency of thealternating current system, the noise pulses occur in a particularunchanging timing pattern, that is to say periodically.

[0007] It is, therefore, the object of the invention to specify asuitable data transmission method which, in particular, can be used indirect-voltage power supplies, and to create an associated device.

[0008] According to the invention, the object is achieved by themeasures as claimed in claim 1. Further developments are specified inthe dependent method claims. A device for carrying out the methodaccording to the invention is the subject matter of claim 11. Furtherdevelopments of the device are specified in the dependent matter claims.

[0009] In the invention, the problem mentioned initially is solved in asimple manner, e.g. by designing the OFDM method in such a manner thatthe OFDM bursts to be transmitted have a length of about 85% of thedistance between two periodic noise pulses. According to the invention,this is possible due to the fact that the defined position of the noisepulses is utilized for synchronizing OFDM transmitter and OFDMreceivers. To ensure that the carriers are orthogonal, the frequencydifference of two carriers must be in an integral ratio to thereciprocal length of the OFDM burst.

[0010] For example, the OFDM bursts will have to have a length of 85%*1.67 ms=1.4 ms in a direct-voltage power supply which is fed by a12-pulse rectifier. This results in a carrier spacing of about 700 Hz sothat, e.g. 32frequencies are available in a bandwidth of 22 kHz. Thus,64 bits can be transmitted per OFDM burst with a QPSK (quadrature phaseshift key) modulation.

[0011] The position of the transmitted OFDM bursts is synchronized withthe pulse-shaped periodic interferers in such a manner that an OFDMburst is located precisely between two noise pulses and is thustransmitted within an almost interference-free time interval. Thepulse-shaped interferers at the receiver are gated out by a suitablecircuit but the OFDM signal located between the interferers can passunimpededly.

[0012] It is suitable to use the first OFDM burst for synchronizing thereceiver in that it contains the so-called preamble. After that,so-called training sequences with a defined content are then in eachcase optionally transmitted in an OFDM burst. This is followed by theOFDM bursts carrying the user information.

[0013] The invention can be advantageously used in traffic engineering,particularly in the case of underground rail systems, municipalrailroads or streetcars which are operated with direct voltage. Inprinciple, however, the use of the method is not restricted todirect-voltage systems of local traffic systems. The method according tothe invention for reducing interference in the data transmission onpower supply systems can be used whenever a power supply system exhibitsperiodic interference. This generally applies to direct voltage systemsand frequently also to alternating-voltage systems.

[0014] Further details and advantages of the invention are found in thesubsequent description of the figures, referring to the drawing, inconjunction with the claims. In the figures:

[0015]FIG. 1 shows a graphical representation of the variation with timeof a direct voltage for an underground rail system,

[0016]FIG. 2 shows the transmitter section and

[0017]FIG. 3 shows the receiver section of a suitable device fortransmitting data with direct-voltage variations according to FIG. 1.

[0018] On power supply lines of underground rail systems, it is intendedto transmit data in addition to the power supply. The technology, alsocalled power line communication (PLC) modulates the information to betransmitted onto suitable carriers and superimposes the modulationproducts on the supply voltage of the underground rail system.

[0019] Underground rail systems are operated, for example, with anominal direct voltage of 750 V. It is general practice to provide thedirect voltage by means of 12-pulse rectifiers which, in turn, are fedby a power converter transformer. The 12-pulse rectifiers consist of two6-pulse rectifiers which, in turn, are fed by two windings of the powerconverter transformer which are electrically offset from one another by30°. The power converter transformer is connected on the primary sidewith the general power supply system with a system frequency of 50 Hz.The method described can also be used at other frequencies such as, e.g.60 Hz, without restriction.

[0020] In general, an underground rail system contains a number ofrectifiers which are installed at intervals of approx. 2 km along therail network. All rectifiers feed the underground rail system jointlyand are electrically connected to one another via the power rail of theunderground rail system.

[0021] To further clarify the problem, FIG. 1 shows by way of example ameasured variation over time of the rail direct voltage over a period of20 ms. The voltage signal is designated by 1. It can be seen that thedirect voltage is not constant but, instead, is subject to fluctuationsof up to approx. 80 V.

[0022] It is particularly during the commutation of the currents in therectifiers that steep-edged voltage jumps with amplitudes of typically50 V occur at intervals of 20 ms/12=1.67 ms (with a 50 Hz supply). Theresultant wide band interference extends up to frequencies of some 100kHz and is thus within the frequency range of power line communicationsystems.

[0023]FIG. 1 shows that the interference caused by the steep-edgedvoltage jumps of the rail voltage can only be partially suppressed byfilters since a part of the spectral noise energy can always passthrough the band-pass. Thus even elaborate filter circuits only reducethe voltage jumps in the rail voltage to 10% and are thus typically 5 V.Thus, there are always still periodic pulse interferers withconsiderable amplitude, which are synchronous with the power system,following a filter.

[0024] The useful signal of a PLC system which arrives at the receiveris dependent on the transmission characteristics of the link and thedistance of the transmitter and is only some 10 mV up to a maximum of 1V because of the limited permissible transmitting power. Thepulse-shaped interference signal is thus always higher than the usefulsignal which results in a very poor signal-to-noise ratio.

[0025] On the one hand, the receiver must be designed for the amplitudeof the interference signals because overdriving the receiver causesdistortion and thus corruption in the received information. On the otherhand, the different levels of interference signal and useful signal makehigh demands on the dynamic range of the receiver. Thus, a resolution of9 bits is necessary just for discriminating between a useful signal of10 mV and a noise signal of 5 V. If the useful signal itself is still tobe received with a resolution of 8 bits, the receiver must have a totalresolution of 17 bits. This is not possible, or only possible with veryhigh expenditure, with commercially available components and at therequired speed.

[0026] Furthermore, the use of automatic gain control (AGC), normallyused in PLC systems and also necessary, is considerably impeded orbecomes impossible due to the interferers.

[0027]FIG. 2 shows a transmitter 20 which consists of an OFDM processingunit 21 followed by an amplifier 22. Using a threshold switch, thedirect voltage is separated and the noise pulses are detected andsupplied as a rectangular reference signal to a phase-locked loop (PLL)which can be of analog or preferably also of digital construction. Thephase-locked loop has the task of suppressing high-frequency jitter sothat the OFDM processing unit receives a stable synchronization signalin the timing pattern of the noise pulses in order to place data packetsprecisely between the pulses. After the PLL has locked to the pulseinterferer sequence, it is advantageous to deactivate the thresholdswitch during the emission of the data and to open a suitable timewindow only at the times at which a noise pulse is expected so that thePLL is not influenced by its own transmit signals which can attain highamplitudes immediately at the transmitter output, and is only fixed tothe noise pulse pattern.

[0028] This is followed by a coupling unit 24 for coupling to a railsystem 25 of an underground rail system. It is also possible to useanother power supply system for traffic-engineering facilities.

[0029] In the transmitter 20, the data signal is partitioned in such amanner that an individual data burst in each case fits between two noisepulses with a certain safety margin. The data signal is thus alwaysemitted in an interference-free time interval and the precise timingcorrelation to the noise pulse pattern being established with thecombination of threshold switch 33 a and subsequent phase-locked loop 33b, described above.

[0030]FIG. 3 shows a receiver 30 of a coupling unit 31 for the railsystem of FIG. 2, which is followed by a filter 32, an impedance 34 anda short-circuiting device 35.

[0031] The short-circuiting device 35 is followed by a unit forautomatic gain control (AGC) according to the prior art. The data arefed via an A/D converter 37 to a processing unit 38 for OFDM signals.

[0032] Here, too, the noise pulses are detected with the aid of athreshold switch at the power system side and supplied as a rectangularreference signal to a phase-locked loop (PLL) for the purpose of jittersuppression.

[0033] The stable synchronization signal from the phase-locked loop inthe timing pattern of the noise pulses is then supplied, on the onehand, to the short-circuiting device 35 and, on the other hand, to theprocessing unit 38 for OFDM signals. The short-circuiting device thussuppresses the received signal for the exact period of a noise pulse andenables the receiver input as soon as the interference-free timeinterval between the noise pulses begins. Due to the stablesynchronization signal, the processing unit 38 for OFDM signals knowsthe exact position in time of the data packets to be processed so thatthere can be a correct data recovery.

[0034] Instead of the short-circuiting device 35 in the output line ofthe filter 32 with preceding impedance 34, a switch in the parallelbranch can also be used. The impedance 34 is not needed in this case.

[0035] After having been decoupled via the impedance 34, the noise pulseis kept away from the OFDM processing unit, e.g. due to the fact thatthe signal line is short-circuited via an analog switch, e.g. in theform of a transistor. As mentioned, it is also possible to seriallymount an electronic switch and to separate the subsequent signalprocessing from the filter output during the pulse interference.

[0036] The signal is supplied to the automatic gain control (AGC)amplifier 36 which amplifies it to a level specified to be optimum forthe A/D conversion. The partition signals are then combined in thesubsequent processing units and decoded in accordance with the priorart.

[0037] The essential factor in the method described above, and theassociated device, is that the OFDM method, known per se, is modified bya partitioning of the data, in such a manner that transmission onlytakes place in almost interference-free time intervals. For thispurpose, the transmitting process is synchronized with the periodicpulse interferer and transmission takes place in each case exactlybetween the noise pulses. Correspondingly, the noise pulses are detectedand gated out in the receiver.

[0038] After the OFDM signal processing, it is possible to assemble thepartitioned data.

[0039] The invention was described above especially for datatransmission in the case of a direct-voltage supply for underground railsystems. The invention can also be used without other power systemsoperated with direct voltage, for example with direct-voltage powersystems for the independent supply of switchgear systems.

1. A method for transmitting data on at least one electrical powersupply line, on which pulse-shaped interference signals withintermediate almost interference-free time intervals occur periodically,using the OFDM (orthogonal frequency division multiplexing) method,wherein the information to be transmitted is distributed over a numberof carriers and the composite signal of all modulated carriers istransmitted in the form of an OFDM burst, having the following features:the OFDM method is designed in such a manner that the OFDM bursts to betransmitted fill up a large proportion of the duration of a period ofthe pulse-shaped interference signals and between two successive OFDMbursts, a pause of the approximate duration of a noise pulse ismaintained, the position of the transmitted OFDM burst is synchronizedwith the pulse-shaped periodic interference signals due to the fact thatthe periodic interference signals represent the synchronization pattern,in each case a fixed-length OFDM burst being located between twoperiodic noise pulses.
 2. The method as claimed in claim 1,characterized in that the OFDM bursts fill up a length of about 85% ofthe duration of the period of the pulse-shaped interference signals andmaintain a pause of approx. 15% of the duration of the period of theinterference signals between two successive OFDM bursts.
 3. The methodas claimed in claim 1, characterized in that synchronization between theperiodically repeated noise pulses and the transmitters and receivers isperformed via threshold switches for pulse detection.
 4. The method asclaimed in claim 3, characterized in that in the transmitter, after thephase-locked loop has locked to the sequence of pulse interferers, thethreshold switch is deactivated during the emission of the data and asuitable time window is only opened at the times at which a noise pulseis expected.
 5. The method as claimed in claim 3, characterized in thatan analog or digital phase-locked loop is used for suppressing jitterduring the synchronization.
 6. The method as claimed in claim 1 andclaim 2, characterized in that the OFDM signal can pass unimpededthrough the circuit for gating out noise pulses.
 7. The method asclaimed in claim 1 and claim 2, characterized in that an impedance isapplied between the coupling unit and the receiver, and the interferersare gated out by short-circuiting the receiving line following theimpedance.
 8. The method as claimed in claim 1 and claim 2,characterized in that the noise pulses are gated out by separating thecoupling unit from the receiver.
 9. The method as claimed in one of thepreceding claims, characterized in that the periodically repeated noisepulses are used for coarse synchronization of OFDM transmitters and OFDMreceivers.
 10. The method as claimed in one of the preceding claims,characterized in that the first OFDM block is used for finesynchronization of the receiver and that, following the finesynchronization, training sequences are transmitted and in that theseare followed by the OFDM bursts carrying the useful information.
 11. Adevice for carrying out the method as claimed in claim 1 or one ofclaims 2 to 10, comprising a transmitter and a receiver, characterizedin that the transmitter (20) and the receiver (30) have processing units(21, 38) for the transmit signals, transmitted in the form of OFDMbursts, with associated coupling units (24, 31) and in that there aremeans (23, 33) for synchronizing the transmit signals with thepulse-shaped periodic interference signals.
 12. The device as claimed inclaim 11, characterized in that the means for synchronizing arethreshold switches (23 a, 33 a) followed by a phase-locked loop (23 b,33 b) in the transmitter (20) and in the receiver (30), the thresholdswitch in the transmitter, after the phase-locked loop has locked, onlybeing active in the time intervals in which a noise pulse is expected.13. The device as claimed in claim 11, characterized in that in thetransmitter (20), an amplifier (22) follows the processing unit (21) forthe OFDM signal.
 14. The device as claimed in claim 11, characterized inthat the receiver (30) has a processing unit (38) for the OFDM signaland a coupling unit (31).
 15. The device as claimed in claim 14,characterized in that the coupling unit (31) is followed by a filter(32), an impedance (34) and a short-circuiting device (35).
 16. Thedevice as claimed in claim 14, characterized in that the coupling unit(31) is followed by a filter (32) and a switch in the longitudinalbranch without impedance.